1. Field of the Invention
The present invention relates to an image production apparatus in which a latent image is formed on an image carrying member and toner is attached on the latent image so that the latent image is developed.
2. Description of the Related Art
In an image production apparatus of an electrophotographic system, there is known an image production apparatus using an endless belt as a photoconductive member constituting an image carrying member, an intermediate transfer member constituting a transfer medium or a conveying device of a paper sheet constituting a transfer medium. The belt is supported by means of a plurality of rollers and driven to be circulated. At this time, a meandering phenomenon that the belt is moved in the direction (main scanning direction) perpendicular to the conveying direction of the belt occurs occasionally.
When such a meandering phenomenon occurs, a position of the latent image formed on the photoconductive member and a position of the image produced on the transfer medium such as the intermediate transfer member and the paper sheet are varied and this variation distorts the image. Further, in a so-called tandem type color image production apparatus in which each of monochromatic images of black (hereinafter abbreviated as K), yellow (hereinafter abbreviated as Y), magenta (hereinafter abbreviated as M) and cyan (hereinafter abbreviated as C) is formed by a single image production unit and these monochromatic images are overlapped one another on the transfer medium to produce a color image, the variation among the image formation positions appear as variation in color among color toner images. The variation deteriorates the image quality and accordingly it is necessary to take any measures to the meandering of the belt in order to produce the high-quality image.
In order to cope with the above problem, various methods are proposed. These methods include, when broadly divided, a method of suppressing the meandering of the belt by means of the passive control, a method of suppressing the meandering of the belt by means of the active control and a method of detecting a meandering amount of the belt and correcting the image formation position in accordance with the detected meandering amount.
As the method of suppressing the meandering of the belt by means of the passive control, for example, JP-A-5-319611 discloses an edge guide provided in one of driven rollers so that thrust force is produced by a rib disposed in the roller to move the belt toward the edge guide and abut an edge of the belt against the guide by the thrust force to thereby prevent the meandering of the belt.
As the method of suppressing the meandering of the belt by means of the active control, for example, JP-A-3-288167, JP-A-8-217302 and JP-A-10-139202 disclose a meandering suppressing method using a so-called active steering system in which one of rollers for supporting the belt is caused to be swung so that the moving direction of the belt is controlled.
The method of detecting a meandering amount of the belt and correcting the image formation position in accordance with the detected meandering amount is disclosed in JP-A-9-222827, in which a predetermined allowable value is defined for the meandering amount and a correction amount is calculated from the meandering amount detected by the meandering amount detecting means so that a writing timing of the latent image by a laser is changed to thereby correct the writing position of the latent image in the main scanning direction.
However, when high-speed image production is considered, the above related art has the following problems.
In the technique of suppressing the meandering of the belt by means of the passive control, the edge of the belt is pressed on the edge guide and according when the belt is moved at a high speed, a large external force is exerted on the end portion of the belt so that the belt tends to be bent and broken. Further, each apparatus requires individual adjustment for differences in movement or behavior of individual belts due to a shape of the end portion of the belt, an assembly accuracy of the apparatus and the like.
In the technique of suppressing the meandering of the belt by means of the active control, when a moving speed of the belt is increased, it is necessary to perform detection and control of the meandering amount with high accuracy, so that configuration of the apparatus is complicated and a cost thereof is increased. Further, when a sudden disturbance such as variation in burden produced due to change in friction depending on entering of a paper sheet, operation of a cleaning device for an image carrying member and density of a toner image is added to the apparatus, it is difficult to perfectly suppress the meandering of the belt caused by such a sudden disturbance.
In the technique of detecting the meandering amount of the belt and correcting the image formation position in accordance with the detected meandering amount, it is essential to detect the meandering amount with high accuracy, so that the cost is increased. Further, it is necessary to make correction in consideration of the meandering amount of the belt upon transfer of a toner image in writing of a latent image, while the meandering of the belt is an irregular phenomenon generally and accordingly it is substantially impossible to forecast the meandering amount perfectly. This problem is more conspicuous in an actual image production process in which the disturbance is not avoided.
As described above, since it is difficult to forecast the movement of the belt in the main scanning direction exactly, the image formation position on the transfer medium is varied and the image quality is deteriorated.
It is an object of the present invention to provide an image production apparatus having an endless belt as an image carrying member and which produces an image at a high speed while preventing deterioration of the image quality due to movement (meandering of belt) in the main scanning direction of a belt member.
An image production apparatus according to the present invention includes at least one image production unit having a latent image forming device for forming a latent image on an image carrying member and a developing device for attaching toner to the latent image formed by the latent image forming device and a toner image formed by the image production unit is transferred to a transfer medium to form an image. Further, the image production apparatus includes a periodicity giving unit for giving periodicity to movement in a main scanning direction of the image carrying member and an amplitude of the period given by the periodicity giving unit is larger than a variation amount in the main scanning direction for determining reversal of the movement in the main scanning direction of the image carrying member.
The periodicity giving unit gives the periodicity to the movement in the main scanning direction of the image carrying member or the transfer medium. When the image carrying member or the transfer medium is structured by an endless belt, there are provided a belt position detector for detecting a position in the main scanning direction of the endless belt, a roller having a rotatable shaft held swingably, a roller swinging mechanism for swinging the rotatable shaft of the roller and a control unit for controlling the roller swinging mechanism so that the movement in the main scanning direction of the belt becomes periodic on the basis of the result of the belt position detector. An amplitude of the period of the endless belt is larger than a variation amount of the belt for determining control for reversing the roller swinging mechanism.
Further, the image production apparatus of the present invention may include a writing position control unit for correcting a write position to the image carrying member of the latent image writing device in accordance with the period of the movement in the main scanning direction of the endless belt.
An image production method of the present invention comprises writing a latent image to an image carrying member by means of a latent image writing device, developing the latent image with toner to form a toner image, detecting a positional variation in a main scanning direction of an endless belt on which the latent image or toner image is formed when the toner image is transferred to a printing medium, giving periodicity to movement in the main scanning direction of the endless belt on the basis of the detected result, and correcting a latent image formation position with respect to the image carrying member on the basis of the periodicity.